Heretofore, gate valves utilizing a reciprocating gate member have included a cast body member having the through-bore and a valve seat therein surrounding the through-bore and a bonnet attached to the body member which provides a chamber opened to the through-bore and which contains the rising or non-rising valve stem for the gate member and space for the gate member when the valve is open. Such prior gate valve structures are of the type shown in the common Assignee's prior U.S. Pat. Nos. 3,633,873, issued Jan. 11, 1972 to Leopold, Jr. et al; 3,662,778, issued May 16, 1972 to Leopold, Jr. et al; and 3,763,880, issued Oct. 9, 1973 to Leopold, Jr. et al. In the gate valves of the aforementioned patents, the valve seat and the gate member were so designed that when the gate member was in the fully opened position, the through-bore was completely unobstructed, thereby reducing turbulence and currents within the valve structure and providing for smoother flow with less pressure drop. However, the machining of the valve seat about the through-bore with the bottom of the valve seat merging with the through-bore is an expensive operation and does provide some manufacturing difficulties as the machining has to be accomplished either through the opening in the upper part of the casing or through the ports of the through-bore.
In some instances, gate valve structures of the types utilizing a valve body and a bonnet arrangement provided separate seats positioned in the valve body for mating with the gate member. Usually, these arrangements resulted in the through-bore through the valve body being partially obstructed by the inserted valve seats, thus causing turbulence or undesirable currents within the valve body. While the gate valves of the aforementioned patents eliminated such problems, they are costly to manufacture and close tolerance has to be maintained between the valve seat and the gate member.
Efforts have also been made to design gate valve structures wherein the valve casing is split into two or more parts in a plane normal to the axis of the through-bore, but again, difficulties have been encountered in providing a gate valve structure wherein there is smooth, unobstructed flow through the valve seat when opened and which had operating mechanism requiring minimum or low torque for operation. Usually, such valves required complicated actuating mechanism rather than a simple rising or non-rising stem or such valves required an extra part to function as the valve seat with the part being held between the two body members of the valve casing. Additionally, such prior art valves did not have all of the functional parts of the valve, such as the valve seat, the gate member, and the valve stem means located in one body member with the other body member functioning primarily as a closure cap. Consequently, it was difficult to provide adequate sealing where the valve stem means extended through both portions of both body members. Sealing between the two body members was difficult since they did not render themselves easy to provide machine surfaces between the body members which could be made to mate with a gasket therebetween for positive sealing.
Gate valve structures in the prior art have been made with gate members having a resilient seal member attached thereto for cooperating with the valve seat. In such prior constructions, the resilient seal member, which was annular, was sometimes positively held in place by another element, but such other element, although it did cause the seal member to expand upon assembly, was bottomed against the gate member so that the pressure on the seal member could not be adjusted once the seal member was positioned on the gate member. These arrangements presented difficulties in adjusting a gate member for sealing with a particular valve seat as there was always tolerance encountered in the manufacturing or machining of the valve seat. Additionally, such arrangements did not provide adjustment of pressure on the seal member at discrete circumferential positions about the same to accommodate for the wear on the seal member once the valve structure had been installed into a fluid distribution system.